1. Field of the Invention
The present invention relates to a light-sensitive lithographic printing plate element, and more particularly, to a light-sensitive lithographic printing plate element comprising a support having thereon a layer of a light-sensitive composition whose adhesion to the support varies upon exposure which is capable of forming an image for lithographic printing by peeling apart development.
2. Description of the Prior Art
Recently, numerous investigations on the preparation of lithographic printing plates have been made and the efficiency of lithographic printing plates produced have been remarkably improved. In particular, the marked improvement of lithographic printing plates in printability and press life has resulted in a change from relief printing used in the past to lithographic printing. Reasons resulting in such a situation are based on various advantages that lithographic printing plates recently have enabled a vast amount of printing to be accomplished due to improvements in printability, the facility with which the plate can be produced and used, the period of time for preparing a printing plate is extremely short, and lithographic printing plates have excellent development stability and plate reproducibility, and in addition, lithographic printing plates have good shelf-life, and the like.
Various lithographic printing plate elements are known in the art. The advantages as mentioned above are achieved in particular with a light-sensitive printing plate element using a photopolymer as a light-sensitive layer.
Typical photopolymers used in these printing plates include a photopolymer mainly composed of a photocrosslinkable type light-sensitive resin which is exemplified by, for example, a polyester or p-phenylene diacrylate and 1,4-cyclohexane diol as described in Belgian Pat. No. 696,533, the reaction products of a phenoxy resin and cinnamic acid or a carboxylic acid having unsaturated groups as described in U.S. Pat. No. 3,387,976, etc. Further, a system comprising as a main component a photopolymerizable material represented by the combination of, for example, diethylene glycol diacrylate, triethylene glycol dimethacrylate, pentaerythritol triacrylate, etc., and a binder such as a resin, e.g., a methyl methacrylate/methacrylic acid copolymer, a styrene/itaconic acid copolymer, etc., is known.
Light-sensitive lithographic printing plates which comprise a coating of these photopolymers are imagewise exposed through an original having a desired negative image and then developed with an appropriate developing solution such as an organic solvent, an alkaline aqueous solution, etc., whereby the areas hardened by irradiation with ultraviolet light remain on the support of the plate and the unexposed areas are dissolved out and removed with the developing solution, and thus a master pattern or dots for printing are formed on the plate support to produce lithographic printing plates.
However, the above-described solution-development type lithographic printing plate elements are becoming extremely disadvantageous not only because complicated treatment steps are required because a developing solution must be used, but also from a viewpoint of sanitary working conditions, hazards, air pollution and environmental pollution which might arise in discharge of waste liquids such as alkali solutions or organic solvents, e.g., developing solutions.
Recently, recording elements which can be developed in a dry manner have been proposed instead of printing plate elements requiring solution development. Of such elements, those as described in Japanese Patent Publication 9663/64, Japanese Patent Publication 22901/68, Japanese Patent Application (OPI) 7728/72 and Japanese Patent Application (OPI) 46315/75 utilize the photopolymerizable property of a photopolymer and the change thereof in adhesion. According to this approach, in general, a layer of a photopolymerizable composition containing as main components a polymer as a binder, an unsaturated monomer and an initiator for photopolymerization is provided on a support such as a synthetic resin film, a metal, paper, etc., and a thin transparent film is further superposed thereon as a cover. The method of making a printing plate using such a light-sensitive printing plate comprises exposing the element through an original and the cover and then peeling apart the cover, whereby either one of the exposed areas and unexposed areas on the light-sensitive layer remains on the support and the other remains on the cover. Thus, a negative image and a positive image (or a positive image and a negative image) are formed simultaneously on the support and on the cover, respectively.
However, in light-sensitive elements of the type which are developed by peeling apart the cover film after exposure, where a hydrophilic support for a lithographic printing plate is provided in direct contact with a layer of a light-sensitive composition in accordance with the conventional technique, the following disadvantages occur.
(1) Upon the formation of an image, after imagewise exposure, by peeling apart the areas having stronger adhesion to the cover film on a light-sensitive layer, together with the cover film, a part of the areas to be peeled apart together with the cover film remains on the hydrophilic support. Where such is then used as a printing plate for lithographic printing, printing ink adheres even to the non-image areas so that the non-image areas on the printed material obtained are scummed and thus it is not practically usable. This phenomenon appears to a marked extent in a so-called negative type light-sensitive material in which an image is formed by peeling apart, after imagewise exposure, the unhardened areas of the light-sensitive layer which are not exposed, together with the cover film to remove them and leaving the exposed hardened areas on the support. In this case, scumming is due to the partial adherence of the unhardened areas to the hydrophilic support.
(2) Because an oleophilic light-sensitive composition is directly provided onto a hydrophilic support, the light-sensitive composition layer has poor stability with the passage of time depending upon the kind of components contained in the light-sensitive composition. Several weeks after the preparation, it is sometimes difficult to obtain a normal image by peeling-apart development.
(3) When a peeling-apart development type lithographic printing plate is stored for a long period of time, innumerable "scum-like spots" are often formed at the non-line image areas after imagewise exposure and peeling-apart development. This is believed to be because, since the peeling-apart development type light-sensitive composition contains hydrophobic liquid components such as ethylenically unsaturated monomers, etc., in most cases, liquid components such as monomers, etc., these components permeate into the grains or pores on the surface of the hydrophilic support for lithography with the passage of time and cause countless spot-like scum areas on the surface of the hydrophilic support where the composition is applied to the hydrophilic support. The non-line image areas should essentially be hydrophilic, but the non-line image areas on which such scum are formed are no longer hydrophilic. Upon printing, heavy scumming (scumming over the entire area) occurs so that it is unsuitable for use as a lithographic printing plate.
With respect to (1) above, when the light-sensitive composition remains even on the non-image areas, such is a serious defect since a basic and necessary requirement from the viewpoint of efficient use of lithographic printing plates is for the non-image areas to possess a hydrophilic surface.
With respect to (2) above, light-sensitive lithographic printing plate elements which have poor stability with the passage of time are by no means practical.
With respect to (3) above, after extensive studies on peeling-apart development type light-sensitive lithographic printing plates were conducted, it was found no special problem arose with a peeling-apart type lithographic printing plate where liquid components had permeated into the grains or pores on the surface of the hydrophilic support, from the standpoint of the formation of desired line images for printing using the so-called peeling-apart development which comprises normal imagewise exposure and removing the unexposed areas together with a peeling-apart sheet. What is a problem, however, is spot-like scum areas which are present on the non-line image areas after peeling-apart development. Where this printing plate is to be used for printing, heavy scumming occurs so that the printing plate is not practically usable. In addition, it is extremely difficult to remove the liquid components which have permeated into the support with any known counter-etch solution or surface treating solution for lithographic printing once the aforementioned liquid components permeate into the grains or pores on the surface of the hydrophilic support.
In any lithographic printing plate, an important characteristic which the lithographic printing plate should have is for the printing plate to be resistant to scumming in the background areas (hereinafter background) or scumming in the non-line image areas, i.e., scumming. Even if no scumming is observed immediately after preparation of the printing plate but such becomes scummed after a period of time has lapsed after preparation, the plate cannot be employed practically.